This invention relates to the drilling and servicing of wells and more particularly to aqueous well completion and workover fluids for use in drilling wells into permeable fluid producing formations and for servicing wells drilled into such formations. In accordance with another aspect, this invention is concerned with the addition of adjuvants to stabilize water loss control agents in wellbore workover fluids, especially at elevated temperatures.
In the servicing of wells drilled into subterranean formations, clear water and various brines and viscous aqueous solutions have been proposed as well completion and workover fluids. These fluids generally do not possess the requisite properties of density, viscosity, gel strength, stability and low fluid loss desired for these applications. Hence, need exists for a non-damaging well completion workover fluid having the requisite properties for use in completing wells and drilling through permeable strata and in conducting workover and similar operations in such wells but which will not result in any substantial permanent damage to the permeable strata which it contacts.
Accordingly, it is an object of this invention to provide an improved well completion and workover fluid. Another object of this invention is to provide a substantially clay-free well completion and workover fluid.
Another object of this invention is to provide well completion and workover fluids which exhibit low fluid loss properties.
Another object of this invention is to provide well completion and workover fluids which exhibit adequate particle carrying capacity as reflected in desirable gel strengths, especially after thermal aging.
Other objects, aspects, as well as the several advantages of the invention will be apparent upon reading the specification and the appended claims.
In accordance with the invention, the water loss property of a clay-free wellbore workover fluid is improved by the addition of an adjuvant selected from (a) sodium bicarbonate (NaHCO.sub.3) (b) sodium bicarbonate (NaHCO.sub.3)+sodium carbonate (Na.sub.2 CO.sub.3) and (c) sodium carbonate+an organic polycarboxylic acid or polycarboxylic acid anhydride.
In accordance with a specific embodiment of the invention, the water-loss property of a clay-free wellbore workover fluid comprising water, an electrolyte such as sodium chloride, an acid-soluble weighting agent such as calcium carbonate, a suspending agent such as asbestos, a polymeric viscosifier such as carboxymethyl cellulose and alkaline reagent such as Na.sub.2 CO.sub.3, is improved by the addition of NaHCO.sub.3 or selected organic carboxylic acids and optionally by the addition of NaHCO.sub.3 alone in the absence of Na.sub.2 CO.sub.3.
The use of sodium bicarbonate adjuvant alone in the inventive compositions is effective at a concentration of at least 10 lbs/bbl of total composition. In compositions for high temperature water loss control with both sodium carbonate and sodium bicarbonate as stabilizing adjuvants, the weight percent of sodium carbonate varies from 65 to 15 whereas the weight percent of sodium bicarbonate varies from 35 to 85 based on the combined weight of Na.sub.2 CO.sub.3 and NaHCO.sub.3 in the inventive composition. The total weight of Na.sub.2 CO.sub.3 and NaHCO.sub.3 in lb/bbl varies over the range of 8 to 20 preferably over the range of 10 to 17. It is contemplated that the adjuvants enhance the water loss control performance, e.g., of carboxymethyl cellulose (CMC) by thermal stabilization of said CMC.
In a further embodiment of the invention, organic polycarboxylic acids are used in combination with sodium carbonate to improve the performance of water loss agents in workover fluids. The polycarboxylic acids can range in the number of carbon atoms from 2 to at least about 76 carbon atoms. Representative examples of suitable polycarboxylic acids that can be used include tartaric acid, citric acid, oxalic acid, tannic acid, adipic acid, phthalic acid, and/or phthalic anhydride and the like and mixtures thereof. The total weight of Na.sub.2 CO.sub.3 and organic polycarboxylic acid in lb/bbl varies over the range of 5 to 10 preferably 6 to 9 with the proviso that the weight ratio of Na.sub.2 CO.sub.3 :polycarboxylic acid varies over the range of 20:1 to 5:1. Presumably, suitable anhydrides of polycarboxylic acids are hydrolyzed in the alkaline environment to polycarboxylic acids.
The electrolyte can be water soluble inorganic salts such as halide and nitrate salts of sodium and potassium together with ammonium salts including sodium chloride, potassium chloride, sodium bromide, potassium bromide, sodium nitrate, ammonium nitrate, ammonium chloride, ammonium bromide and the like and mixtures thereof. Saturated sodium chloride solution is presently preferred as the base fluid in the inventive compositions which exhibit densities in the range of 13.5 to 15 lb/gal after the addition of selected acid-soluble weighting agents. Sea water can be used for the preparation of workover fluids with densities in the range of 11-12 lb/gal. Thus the instant invention is applicable to improving the water loss properties of workover fluids having densities in the range of about 11 to about 15 lb/gal.
Weighting agents which are completely acid-soluble, such as CaCO.sub.3, BaCO.sub.3 and iron carbonate, are preferred in the inventive compositions to give densities in the above cited range. These additives are completely acid-soluble and can be dissolved and back-flushed with acid from subterranean formations to prevent formation damage or plugging. Presumably such undesirable formation damage can occur with workover operations using fluids comprising nonacid-soluble weighting agents such as barium sulfate. Ideally, a workover fluid should contain no solids, however, the addition of insoluble weighting agents is frequently necessary to raise fluid density to the desired level. Ferric and ferrous oxides can also be used as weighting agents in the inventive compositions. Ferrous oxide is preferred over ferric oxide because of the former's greater solubility in 15 percent HCl which is the fluid frequently used to correct formation damage.
Suitable viscosity characteristics can be imparted to the inventive compositions by the use of natural polymer viscosity additives such as guar gum, cellulose ethers, polysaccharides, and the like, and mixtures thereof. With these viscosity additives the carrying capacity of the fluid will vary in the agitated and non-agitated states, thus, for example, in a non-agitated separating tank, the accumulated fluid loses its carrying capacity and sand, debris and the like drop out whereas the agitated fluid has sufficient carrying capacity to carry cuttings and the like to the surface should the fluids be used for such purposes. These viscosity-increasing additives also exhibit effectiveness as water-loss agents in general, however, in the present compositions the overall water-loss control property of the viscosity additives is greatly improved by the addition of NaHCO.sub.3 alone or Na.sub.2 CO.sub.3 and NaHCO.sub.3.
The cellulose ethers which can be used include, among others: the various carboxy alkyl cellulose ethers such as carboxyethyl cellulose and carboxymethyl cellulose (CMC); mixed ethers such as carboxyalkyl hydroxyalkyl ethers such as carboxymethyl hydroxyethyl cellulose (CMHEC); hydroxyalkyl celluloses such as hydroxyethyl cellulose and hydroxy propyl cellulose; alkyl hydroxyalkyl celluloses such as methyl hydroxy propyl cellulose; alkyl celluloses such as methyl cellulose, ethyl cellulose and propyl cellulose; alkyl carboxy alkyl celluloses such as ethyl carboxy methyl cellulose; alkyl alkyl celluloses such as methylethyl cellulose; and the like. Many of the cellulose ethers are available commercially in various grades. The carboxy-substituted ethers are available as the alkaline metal salt, usually the sodium salt. However, the metal is seldom referred to and they are commonly referred to as CMC for carboxy methyl cellulose and CMHEC for carboxymethylhydroxyethyl cellulose, etc. A presently preferred cellulose ether is CMC. Water-dispersible CMC is available in various degrees of carboxylate substitutions ranging from about 0.3 up to the maximum degree of substition of 3.0. In general, CMC having a degree of substitution in the range of 0.65 to 1.3 is preferred. CMC having a degree of substitution in the range of 0.85 to 1.2 is a more preferred cellulose ether. CMC having a degree of substitution less than the above-preferred ranges is generally less uniform in properties and thus less desirable. CMC having a degree of substitution greater than the above preferred ranges usually has a lower viscosity and more is required in preparing suitable compositions. The degree of substitution of CMC is commonly designated in practice as CMC-7, CMC-9, CMC-12, etc., where the 7, 9, and 12 refer to a degree of substitution of 0.7, 0.9 and 1.2, respectively.
Another suitable polymeric viscosifier that can be used according to the invention is guar gum. Guar gum is a free-flowing nonionic, water soluble polymer. Guaran, the scientific name for guar gum, is a straight-chain mannan with single-membered galactose branches composed of about 80% D-galacto-D-mannoglycan, 5% proteins, 2% crude fibers, and 1% ash. The ratio of D-galactose to D-mannose in guaran is 35:65. The D-mannopyranose units are joined by (1.fwdarw.4) links, and single D-galactopyranose units are joined to this chain by a (1.fwdarw.6) links. On the average, the galactose branches occur in every other mannose unit. A commercially available guar gum identified as Guar THI sold by Hercules Incorporated is presently preferred.
Suitable polysaccharides include the ionic heteropolysaccharides produced by fermentation of carbohydrates by bacteria of the genus Xanthomonas. Exemplary of such heteropolysaccharides are those produced by Xanthomonas campestris, Xanthomonas begonia, Xanthomonas phaseoli, Xanthomonas hederae, Xanthomonas incanae, Xanthonomas carotae, and Xanthomonas translucens. Of these, ionic polysaccharide B-1459 is preferred. This polysaccharide is prepared by culturing the bacterium Xanthomonas campestris NRRL B-1459, United States Department of Agriculture, on a well-aerated medium containing commercial glucose, organic nitrogen sources, dipotassium hydrogen phosphate, and various trace elements. Fermentation is carried out to completion in 4 days or less at a pH of about 7 and a temperature of 28.degree. C. Polysaccharide B-1459 is commercially available under the trade name of "Kelzan" from the Kelco Company, San Diego, Calif.
The amount of polymeric viscosifier used in the practice of the invention can vary widely depending upon the viscosity grade and purity of the polymer, and properties desired in compositions of the invention. In general, the amount of polymeric viscosifier used will be a water thickening amount, i.e., at least an amount which will significantly thicken the water to which it is added. In general, amounts in the range of about 0.25 lb/bbl to about 3 lb/bbl can be advantageously employed and with preferred amounts based primarily upon economics ranging from about 0.75 lb/bbl to about 2 lb/bbl.
Asbestos is used as a suspending agent for the weighting material in the inventive compositions. In this role asbestos is relatively inexpensive and the desirable flow characteristics (rheological properties) of the fluid systems are maintained. Of the various types of asbestos which are commercially available the asbestos derived from chrysotile is presently preferred. The chrysotile asbestos fibers provide maximum carrying or suspending properties with a minimum of asbestos. The following examples further demonstrate the operability of the instant compositions.
The completion and workover fluid compositions of this invention are prepared by admixing the desired proportion of the various ingredients with water. All of the ingredients are fairly readily dissolved or dispersed in water by circulation through the conventional mixing equipment of a rotary drilling rig.